Interfacially dominated giant magnetoresistance in FeÕCr superlattices J. Santamaria, 1, * M.-E. Gomez, 1,² M.-C. Cyrille, 1,‡ C. Leighton, 1 Kannan M. Krishnan, 2 and Ivan K. Schuller 1 1 Department of Physics, University of California – San Diego, La Jolla, California 92093-0319 2 Materials Sciences Division, National Center for Electron Microscopy, Lawrence Berkeley Laboratory, University of California – Berkeley, Berkeley, California 94720 ~Received 14 August 2001; published 3 December 2001! We have performed an extensive comparative study of growth, structure, magnetization, and magnetotrans- port in Fe/Cr superlattices. A simple analysis of the experimental data shows that the giant magnetoresistance originates from interfacial scattering in the Fe/Cr system. The saturation resistivity is determined by the roughness lateral correlation length whereas the giant magnetoresistance is determined by the interface width. DOI: 10.1103/PhysRevB.65.012412 PACS number~s!: 75.70.Cn, 75.70.Pa Studies of giant magnetoresistance ~GMR! in metallic su- perlattices have produced much new physics since its discovery. 1–3 Most studies of magnetotransport in metallic superlattices are performed with the current parallel to the interfaces @current in the plane ~CIP!# geometry. However, the geometry in which the current flows perpendicular to the interfaces @current perpendicular to the plane ~CPP!# ~Refs. 4–10! is much more amenable to theoretical studies, and has recently produced important applications. 11 To the best of our knowledge there are no experimental studies that connect in a quantitative fashion well-defined structural parameters and magnetotransport. The reasons for this are as follows: CPP measurements are notoriously difficult experimentally, the structural complexity of a superlattice requires detailed- quantitative structural measurements, sample characteristics are delicately dependent on preparation conditions, and the magnetic properties are strikingly affected by small changes in preparation conditions and structural parameters. To ad- dress all these issues we have performed a detailed experi- mental study to investigate the connection between CPP- GMR and structure. To do this we brought together two well- established quantitative structural analysis techniques with a lithography-based CPP measurement technique and magneti- zation on a large set of samples. We find evidence that in Fe/Cr superlattices both the CPP resistivity and the CPP- GMR originate mainly from the interfaces. These results pro- vide well-defined quantitative results that should be key in- gredients in theories dealing with GMR in metallic superlattices. Studies of transport in metallic superlattices are affected by many inherent complexities of the material. Many pos- sible complications arise in these types of artificial materials: a! interfacial roughness and/or interdiffusion at various lat- eral length scales, 12–14 ~b! bulk defects, ~c! structural changes as a function of individual layer and/or overall thickness, ~d! different length scales affecting the structure, magnetism, and transport, and ~e! differences in the magne- totransport along the different directions in the superlattices. Moreover, theoretical treatments of the problem are much more amenable if the current flow is perpendicular to the interfaces of the layers ~CPP!. It is, therefore, desirable to have a study in which the CPP-GMR is directly related to structural parameters independently measured using quanti- tative structural probes. The quantitative determination of all structural parameters in a superlattice is rather difficult since different techniques give information with varying accuracy along different directions ~perpendicular or parallel to the interfaces!. In order to obtain a quantitative description of the superlattice it is useful to cross correlate various mea- surement techniques on samples made under different condi- tions. The measurement of the magnetoresistance is also complicated by the fact that it is desirable to measure inde- pendently the resistivity and the magnetoresistance. The rea- son for this is that these two quantities may be affected in different ways by structural parameters and, therefore, a measurement solely of the ratio of the two quantities may not be sufficient. Moreover, the GMR depends also on the degree of antiferromagnetic ~AF! alignment in the superlattice and, therefore, measurements of the magnetization are also a key ingredient in order to obtain a clear cut answer. A key issue in the mechanism of GMR is the relative importance of bulk and interfacial scattering. This is particu- larly difficult to clarify since in many cases both the bulk and interfacial scattering are affected when layer or overall thick- nesses of the superlattice are varied. Moreover, in the CPP measurements the roughness and interdiffusion are also af- fected by the initial roughness of the electrodes underlying the sample. Due to this, whether the GMR is mostly interfa- cial or bulk in origin is quite controversial. Measurements as a function of layer thickness, analyzed within a particular model have claimed that the GMR originates from the bulk and that interfacial roughness does not play a crucial role. 15 Other measurements in which the interfaces were modified by the addition of small amounts of interfacial impurities, claim that the interfacial scattering plays a dominant role. 16 It may even be possible that the exact mechanism is materials system dependent. It seems that no experiments are available where the role played by ‘‘long-wavelength’’ roughness ~larger than atomic! was investigated. Here we have tackled this problem in a comprehensive fashion. We have made two different types of superlattices by sputtering, where we vary: a! the number of bilayers, and b! sputtering pressure with a fixed number of bilayers. We have characterized the structure of the superlattices using quantitative x-ray diffraction and quantitative energy-filtered transmission electron microscopy ~EFTEM! spectra. We measured the magnetization in order to obtain a quantitative measure of the antiferromagnetically aligned fraction. This is PHYSICAL REVIEW B, VOLUME 65, 012412 0163-1829/2001/65~1!/012412~4!/$20.00 ©2001 The American Physical Society 65 012412-1